The present invention relates to the art of dental restoration. Presently, all dental work is done using the age-old casting/lost wax technique. Efforts are underway to replace this method, as well as direct filling techniques, by utilizing modern technology such as three-dimensional data acquisition and computer controlled milling devices. The applicants' above-mentioned U.S. Pat. No. 4,575,805 teaches such a scheme. Devices have been developed which can machine uniquely shaped items, for example as disclosed in Yamada U.S. Pat. No. 4,385,360 and Duret European publication No. 91 876, which corresponds to Duret U.S. Pat. No. 4,611,288. Becker European publication No. 33 492 which corresponds to U.S. Pat. No. 4,411,626, teaches the fabrication of the desired dental pieces, but requires an intermediate step, namely the machining of a wax model. It should be noted that, to date, the issue of shaping hard dental materials within the time frame set by a dentist's schedule has not been specifically addressed. Grinding machines of the type typically found on a factory floor could well machine the desired materials, for example machines of the type disclosed in Bolender U.S. Pat. No. 2,423,367, Wies German Offenlegungsschrift No. 15 02 515, and Kondou Japanese Pat. No. 57-173447. Such devices all use grinding wheels in one way or another, but none can be adapted for the particular size requirements of a dental implant, nor to shape the implant all over as required according to the present invention. Other systems such as those disclosed in Reid British Pat. No. 1 114 929 and Torit British Pat. No. 891 330 are adapted to do finishing only; in other words the parts are already in a semi-finished form and are merely polished, deflashed or trimmed.
It is therefore an object of the present invention to provide to the dentist a small and affordable unit which allows the fabrication of dental restorative parts in a single session with a patient.
It is a further object to arrange this unit in a way which eliminates manual changing of tools and reclamping of the workpiece or other operator interaction during the machining process.
It is yet another object of the invention to make the system self-calibrating, or in other words to make it automatically compensate for tool wear.
A principal object is to provide a machine which can shape the hard ceramic materials ideally suited to the oral environment in a matter of a few minutes and without the need of extended manual trimming. Initially, an apparatus which utilizes a small diamond burr of the type dentists commonly use in hand-held instruments would seem to fulfill the requirements. However, due to the much larger volume of material to be machined, and the fact that a natural tooth, apart from its enamel exterior, is relatively soft and that in any event the dentist tends to remove mainly decayed material, the diamond burr is not suited for the specific job. Another consideration is the high speed required to drive a tool with a diameter this small. A cavity is typically contoured in a minute or two, and the hand-held turbine can in any event be replaced if excessive wear occurs. In contrast, the invention requires a machine which should ideally be able to operate for many hours without maintenance. While the restoration can be made from metallic alloys, public demand calls for white, tooth-like inlays which restore the tooth both functionally and aesthetically. Ceramic or composite materials (acrylics loaded with inorganic matter) best meet these requirements